Semiconductor devices and the like are produced using a stepping or step-and-scan projection aligner which transfers a pattern of a reticle as a photomask onto each shot region on a wafer coated with a photoresist through a projection optical system.
The resolution of a projection optical system used in a projection aligner increases as the exposure wavelength decreases and the numerical aperture of the projection optical system increases. Therefore, the exposure wavelength (i.e., the wavelength of radiation used in the projection aligner) has decreased and the numerical aperture of the projection optical system has increased year by year as integrated circuits have been scaled down.
The depth of focus is as important as the resolution when a resist is exposed. Resolution R and depth of focus δ are expressed respectively by the following formulae. When obtaining the same resolution R, a larger depth of focus δ can be obtained using radiation with a shorter wavelength.R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)wherein λ is an exposure wavelength, NA is a numerical aperture of the projection optical system, and k1 and k2 are process coefficients.
In the above exposure technology, a photoresist film is formed on the surface of the exposure target wafer, and the pattern is transferred to the photoresist film. In a conventional projection aligner, the space in which the wafer is placed is filled with air or nitrogen.
When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the resolution R and the depth of focus 6 are represented by the following formulae.R=k1·(λ/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
For example, when water is used as the medium in an ArF process, the resolution R is 69.4% (R=k1·(k/1.44)/NA) and the depth of focus is 144% (6=k2·1.44λ/NA2) as compared with the case in which air or nitrogen is used as the medium, when the refractive index of light with a wavelength of 193 nm is n=1.44.
Such a projection exposure method in which the wavelength of radiation is reduced to transfer a more minute pattern is called liquid immersion lithography. The liquid immersion lithography is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several tens of nanometers. A projection aligner used for the method is also known in the art.
In the liquid immersion lithography using water as a medium of immersion, a photoresist film formed on a wafer and a lens of a projection aligner are brought into contact with water. For this reason, water may permeate the photoresist film and decrease resolution. In addition, photoresist components may elute into water and pollute the surface of the lens of the projection aligner.
A method of forming an upper layer film on a photoresist film in order to shut out the medium such as water has been proposed (see, Patent Document 1 for example).
[Patent Document 1] JP-A 2007-24959